More than just vaccines

Flies in space! What fruit flies are teaching us about immunology

Most of us have had a few run-ins with the bane of the kitchen, the fruit fly. Known better by some as Drosophila melanogaster, scientists have been studying the species since the early 1900s. But until college labs, I never understood what you would want to know about them other than how best to keep them out of your bananas.

Believe it or not, research using fruit flies has given us much of our modern understanding of genetics. Research using fruit flies led to the idea that chromosomes contain genes, the concept of mapping where genes are within a chromosome, the fact that ionizing radiation causes genetic mutations and the fact that XX chromosomes makes females female and XY makes males male.

Drosophila melanogaster

Fruit flies have more recently been used to understand human immunology. A study published last month in PlosOne describes immune responses of flies that were born in outer space aboard the shuttle, Discovery. It may sound far-fetched, but fruit flies are a cheap and compact model organism that has a lot in common with humans, especially when it comes to immunity.

I’ve written a lot about the adaptive immune system—mainly B cells and T cells with very specific receptors that recognize very specific parts of infecting pathogens and become memory B and T cells. There is another branch of immune cells that make up the innate immune system. They have receptors that detect general patterns found on pathogens, called (appropriately) pattern recognition receptors. For example, some recognize sugars arranged in repeated patterns on the surface of bacteria. Pattern recognition receptors are less specific than receptors on B and T cells, and don’t generate memory. Instead, they act as a first line of defense that allows immune cells to react quickly and make signaling proteins to direct the immune response toward inflammation.

Toll-like receptors make up the class of innate receptors that kicked off studies into the innate branch of immunity. They were given their name due to their shared genetic sequence with a fruit fly receptor called Toll. Studies in fruit flies gave researchers major clues throughout the early 1990’s about how Toll-like receptors work. (For a complete history of how it all happened, check out this Nature review, or this free article.) Toll-like receptors are found on both adaptive and innate immune cells, as well as neurons and epithelial cells. We are constantly sensing our environment via Toll-like receptors. They are vital for responding to viral and bacterial infections and help keep bacteria in our gut microbiomes under control. Many researchers are looking for the best ways to activate them during vaccination in place of current adjuvants. It may seem odd, and maybe a little bit gross, but without those pesky flies, we would not know nearly as much about how our own immune systems function.

In case you were curious, the flies born on the space shuttle Discovery—called “space flies” by the authors of the paper—were infected with fungal spores and bacteria and the researchers examined changes in genes involved in immune responses. Unfortunately, the “space flies” were overall immunosuppressed and at a disadvantage compared to “earth flies,” leading the authors to conclude that gravity plays a key role in immune function.